Rotating a view in layoutSubviews

The problem with the code in the question seems to be that the transformations keep getting added to each other. In order to fix this, the solution is to reset the transformations every time, that is, set it to the identity transform.

rotationView.transform = CGAffineTransformIdentity

Here is a partial implementation that shows the key parts.

import UIKit
@IBDesignable class UIVerticalTextView: UIView {

    var textView = UITextView()
    let rotationView = UIView()

    var underlyingTextView: UITextView {
        get {
            return textView
        }
        set {
            textView = newValue
        }
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override init(frame: CGRect){
        super.init(frame: frame)
        self.setup()
    }

    override func awakeFromNib() {
        super.awakeFromNib()
        self.setup()
    }

    func setup() {

        rotationView.backgroundColor = UIColor.redColor()
        textView.backgroundColor = UIColor.yellowColor()
        self.addSubview(rotationView)
        rotationView.addSubview(textView)

        // could also do this with auto layout constraints
        textView.frame = rotationView.bounds
    }

    override func layoutSubviews() {
        super.layoutSubviews()

        rotationView.transform = CGAffineTransformIdentity // *** key line ***

        rotationView.frame = CGRect(origin: CGPointZero, size: CGSize(width: self.bounds.height, height: self.bounds.width))
        rotationView.transform = translateRotateFlip()
    }

    func translateRotateFlip() -> CGAffineTransform {

        var transform = CGAffineTransformIdentity

        // translate to new center
        transform = CGAffineTransformTranslate(transform, (self.bounds.width / 2)-(self.bounds.height / 2), (self.bounds.height / 2)-(self.bounds.width / 2))
        // rotate counterclockwise around center
        transform = CGAffineTransformRotate(transform, CGFloat(-M_PI_2))
        // flip vertically
        transform = CGAffineTransformScale(transform, -1, 1)

        return transform
    }
}

My most recent implementation is most likely to be found in this github link.